This invention relates generally to piezoelectric actuators and sensors for use in composite structures of various types, and, more particularly, to techniques for embedding actuators and sensors in composite structures. Structures used in aerospace applications usually have to be stiff, light in weight, and able to dissipate mechanical energy in the form of vibration. Various techniques have been proposed for the dissipation of energy, and these may be categorized as active or passive approaches.
Passive energy dissipation typically uses layers of viscoelastic materials, either embedded in structural members or applied at structurally joints. Viscoelastic materials have the useful property that their stiffness increases with the rate at which they are mechanically stressed. Therefore, a viscoelastic member is subject to relatively high damping forces when vibrating rapidly, and lower damping forces when deformed more slowly. Although significant levels of damping can be obtained by passive means, static strength is reduced by the presence of the viscoelastic materials, since structural members must be reduced in stiffness at certain locations in order to introduce energy into the viscoelastic material. Also, since the properties of viscoelastic materials are highly temperature dependent, a thermal control system must be used in space structures, to maintain them at the desired operating conditions. This is costly in terms of both power consumption and weight, and in some cases thermal control is not possible at all.
The active damping approach employs actuators, which, operating in conjunction with deformation sensors, apply damping forces to compensate for vibrational and other loading forces on structures. Static structural strength and stiffness are not compromised by active damping, and many ceramic actuator materials can retain their properties over a much larger temperature range than passive damping structures. In many cases thermal control systems are not needed if active damping is used.
The principal difficulty with actively damped structures is that of embedding ceramic actuators in composite materials. The most efficient actuator materials are piezoelectric ceramics, such as lead zirconate titanate (PZT), and electrostrictive ceramics, such as lead molybdenum niobate (PMN). Piezoelectric materials can be used as actuators, since a voltage applied to them causes structural deformation in a selected direction, or as sensors, whereby a deformation in a selected direction induces a measurable voltage. Unfortunately, most ceramic materials are relatively brittle and have a large positive coefficient of thermal expansion (CTE). Graphite fiber-reinforced epoxies have a zero or slightly negative CTE. When a graphite fiber reinforced epoxy is cured at elevated temperatures, and subsequently cooled, tensile stresses are induced in the embedded ceramic materials. Resultant cracking will degrade ceramic actuator performance significantly, if not totally. Another difficulty is the need to insulate actuators electrically from conductive graphite fibers in the composite materials in which the actuators are embedded. Ceramic actuators and sensors have two electroplated surfaces with attached connecting leads that must be insulated. Yet another problem is the inherent fragility of the actuators, which may lead to breakage even prior to embedment in a composite structure.
One proposed solution to these problems was to encase an actuator or sensor in a Kapton envelope before embedding it in a structure. The principal drawback of this solution is that the Kapton and the associated adhesive material with which it is supplied, do not form an intimate bond with the ceramic. Consequently, mechanical deformation of the ceramic material is not firmly coupled to the laminated structure in which the envelope is embedded, and, in the case of a sensor, deformation of the structure is not properly transmitted to the ceramic.
It will be appreciated from the foregoing that there is still a need for improvement in the field of piezoelectric ceramic sensors and actuators as applied to active damping of composite structures. The present invention is directed to this end.